1. Technical Field
The present invention relates to a ferroelectric memory in which hysteresis characteristics of remanence polarization of ferroelectric are used for data retention, and more particularly to a nondestructive readout ferroelectric memory.
2. Related Art
It is generally known that a ferroelectric material has a hysteresis characteristic, and a ferroelectric memory using such a characteristic can be formed to store data. Conventionally, in the method of reading these memories, a destructive readout that uses polarization reversal currents, which requires rewriting of a memory cell that is selected to be read out, is performed. In this case, there are problems of lowered readout speed and fatigue due to the existence of rewriting operations, which causes a bottleneck as disadvantage to flash memories. Therefore, nondestructive readout methods have been examined. However, these methods entail a variety of problems, such as, for example, a small difference between “1” and “0”, a process-related reliability problem for example in a method of reading a current difference such as a 1T type, and the like.
There is a problem in that the difference in the signal amount between “1” and “0” is small in a nondestructive readout method. For this reason, readout methods devised as follows have been proposed.
Japanese Laid-open Patent Application HEI 5-55664 describes a method in which a difference in differential permittivities is read out by applying a pulse below Vc. Japanese Laid-open Patent Application HEI 5-129622 describes a method in which the phase of a second harmonic component of a current response is detected by applying a bipolar pulse below Vc. Japanese Laid-open Patent Application HEI 6-275062 describes a method in which the phase of a second harmonic component of a current response is detected by applying a bipolar pulse below Vc.
Among the above-described art, in the method that uses a difference in differential permittivities for data readout (HEI 5-55664), the difference in differential permittivities is extremely small, such that it is extremely difficult to detect it by an ordinary readout method. Also, the difference may be readily cancelled by influences caused by continuous reading cycles, disturbs and the like.
The method in which the phase detection of a difference of harmonic components is performed (HEI 5-129622, HEI 6-275062) has a substantial problem in the readout sensitivity, because it is almost like picking up a minute signal from among noises.
Besides the above, there has been proposed a nondestructive readout method of reading a difference of currents when a minute bias is impressed.
However, the difference of currents upon impressing a minute bias is a difference that originates in the switching of the ferroelectric device, and there is a tendency that the difference is lost with the passage of time. The greater the difference between the initial “1” and “0,” the more remarkable the tendency becomes, and a nondestructive readout is in effect impossible.